Regulating means for rotary piston compressors



Oct. 29, 1963, L. B. SCHIBBYE REGULATING MEANS FOR Roma. PISTON COMPRESSORS Filed June 17, 1960 4 Sheets-Sheet 1 Oct. 29, 1963 L. B. SCHIBBYE REGULATING MEANS FOR ROTARY PISTON COMPRESSORS Filed June 17, 1960 4 sheets-shee 2 L. B. SCHIBBYE Oct. 29, 1963 REGULATING MEANS FOR ROTARY PISTON COMPRESSORS 4 sheets-sheet a Filed June 17, 1960- F1- .7 Byz- Q Filed June 17, 1960 Oct. 29, 1963 B. SCHIBBYE ,1

REGULATING MEANS FOR- ROTARY PISTON COMPRESSORS 4 Sheets-Sheet 4 United States Patent 3,108,740 REGULATING MEANS FUR ROTARY PISTON COMPRESSQRS Lauritz Benedictus Schibbye, Saltsjo-Duvnas, Sweden, assignor to Svenska Rotor Maskiner Alktielaolag, Nacka, Sweden, a corporation of Sweden Filed June 17, 1960, Ser. No. 36,765 21 Claims. (1. 230-138) The present invention relates to rotary piston, positive displacement compressors for compressing elastic fluids, and has particular reference to that type of such compressors disclosed in Nilsson, US. Patent No. 2,622,787, granted December 23, 1952.

Basically it is characteristic of such compressors that compression chamber-s are formed by the intermeshing action of the helical lands and grooves of male and female rotors mounted to rotate about coplanar axes in an appropriate housing structure, with the lands of the male rotor lying substantially outside of the pitch circle of the rotor and having a wrap angle of less than one full turn or less than 360, while the lands of the female rotor likewise have a wrap angle of less than one full turn, lie substantially within the pitch circle of the fe male rotor.

It is further basically characteristic of such compressors that chevron shaped compression chambers are formed by communicating groove portions of the cooperating rotors and the confronting wall portion of the housing structure, the base ends of such chambers being defined by a fixed transverse plane, usually coincident with the high pressure end wall of the housing structure, and the apex ends being defined by the place of intermesh between the rotors, which moves axially away from the inlet ends of the rotors towards said fixed plane to cause the chambers to run out to minimum volume at said plane as the rotors revolve. The fixed plane is located at a place axially spaced from the inlet ends of the rotors where the high pressure or discharge port of the compressor is situated, the port being substantially confined to one side of the plane of the rotor axes, which may conveniently be designated the high pressure side of the compressor.

Similarly, it is basically characteristic of such compressors that on the opposite or low pressure side of the compressor, where the low pressure or inlet port of the compressor communicates with the inlet ends of the rotors, chevron shaped suction chambers are formed be tween rotor groove portions and confronting housing structure, which chambers expand from minimum volume when the places of intermesh defining the apex ends of the chambers are at the inlet ends of the rotors to maximum volume when the places of intermesh have moved axially away from the inlet ends of the rotors to the aforesaid fixed plane and the groove portions forming the chambers have moved out of communication with each other as the rotors revolve.

Compressors of the kind generally described above have been and are being utilized extensively in various fields of commercial application, including many fields in which part load or part capacity operation over extended periods of time is highly desirable. Operation at partial capacity is of course readily accomplished through the simple expedient of throttling the intake of the compressor, but this kind of regulation entails throttling losses with consequent loss of efliciency which makes such a kind of control undesirable for those applications where part load operation may constitute a substantial portion of the total time of operation.

In order to provide for more efiicient operation at partial capacity of compressors of the helical rotor type, than can be accomplished simply by throttling the intake,

3,108,740 Patented Oct. 29, 1963 various forms of valve devices have heretofore been proposed, none of which, however, have proved to be wholly satisfactory from all standpoints even though effecting some improvement in efficiency. Thus for example, ca pacity control valves of the kind disclosed in Lysholm, US. Patent No. 2,459,709, granted January 18, 1949, which are interposed between stationary housing parts and the peripheries of the moving rotors, involve difiiculties of a mechanical nature such as the initial establishment of and the maintaining of the very exact clearances which are critical to the efiicient operation of the machine and also require the presence of a lubricant between surfaces in direct communication with the compression chambers, a condition that could not be tolerated for some uses (in the chemical industry, for example) of so-called dry compressors.

Other prior proposals have, for similar and other reasons, proved to be less than wholly satisfactory and it is therefore the principal and general object of the present invention to provide new and improved regulating means for controlling the operation of compressors of the kind under consideration in a way which will enable them to be operated efiiciently at variable part loads down to a relative small fraction of full load for full capacity. It is a further object to accomplish such regulation by valve means which introduce no throttling effects and losses, and which moreover are not only simple in mechanical construction but also of such nature that critical clearances between stationary and rotating parts are easily established initially and readily maintained over long periods of use. Other and more detailed objects of the invention will become apparent as the ensuing description of suitable embodiments of apparatus for carrying the invention into effect proceeds, with reference to the accompanying drawings, in which;

FIG. 1 is a longitudinal section, partly in elevation, of a suitable embodiment of apparatus for carrying the invention into effect;

FIG. 2 is a section taken on the line 2-2 of FIG. 1;

FIG. 3 is a View similar to FIG. 1 of another embodiment of apparatus embodying the invention;

FIG. 4 is a section taken on the line 4-4 of FIG. 3;

FIG. 5 is a side elevation, partly in section, of still another embodiment of the invention;

FIG. 6 is a section taken on the line 6-6 of FIG. 5;

FIG. 7 is a fragmentary longitudinal section, partly in elevation, of still another embodiment of the invention;

FIG. 8 is a section taken on the line 8-8 of FIG. 7;

FIG. 9 is a view similar to FIG. 7 of still another embodiment of the invention; and

FIG. 10 is a section taken on line 10-10 of FIG. 9.

Referring now to the drawings, and more particularly to FIGS. 1 and 2 thereof, the compressor comprises a stationary housing structure 28 having a barrel portion providing a working space composed of two intersecting cylindrical bores 30 and 32 with coplanar axes which in the embodiment illustrated are parallel. The housing structure is further provided with portions forming end walls 34 and 36 defining the axial length of the working space. A male rotor 20 provided with helical lands 22 is rotatably mounted in bore 30, and a female rotor 24 likewise provided with helical lands 26 is rotatably mounted in bore 32 with the lands of the two rotors in intermeshing engagement. As shown, the lands and grooves of the rotors have a wrap angle of less than one full turn (less than 360), the lands of the female rotor lying substantially entirely within the pitch circle of the rotor while the pitch circle of the male rotor lies substantially at the root circle of the lands of the rotor. Also the lands of the male rotor are convexly curved and of substantially circular profile, while the lands of the female rotor are correspondingly concave, as shown, the construction being advantageously as disclosed in Nilsson, US. Patent No. 2,622,787; granted December 23, 1952.

The rotors are mounted by appropriate bearings 46 at the inlet or low pressure end of the compressor and bearings -48 at the high pressure end, the drive in the embodiment shown being to the male rotor by way of the stub shaft 52 and the female rotor being driven from the male rotor through the medium of suitable timing gears one of which is shown at St) in FIG. 1.

The housing structure provides a main low pressure or inlet channel 42 terminating in a main inlet port 38 located partly in the end wall 34 of the compressor and partly in the barrel portion providing the bores in which the rotors are located. As will be noted from FIG. 2, that part of the port 33 opening radially into the barrel portion of the casing is narrower peripherally than that part of the port opening axially through the end wall 34, the maximum peripheral extent of the port being defined by the port edges 56 and 58 in the end wall, the radial extent of the port being defined by the edge 54 in the end wall. This configuration of the port is important as will hereinafter appear more fully.

Fluid compressed in the apparatus is discharged from the compression chambers through the high pressure or discharge port 40 to the high pressure discharge channel 44, port 40 being located partly in the high pressure end wall 86 and partly in the barrel portion of the compressor at the end thereof opposite from the main inlet port 38.

The rotors are mounted to revolve in the housing with appropriate running clearance between the peripheries thereof and the walls of the bores in the barrel portion of the housing and between the ends thereof and the respective end walls of the housing.

In addition to the main inlet port 33 located in the low pressure end wall 34, the present embodiment of the invention provides a plurality of auxiliary inlet valve means 60 and 62. In this embodiment, these several and separate valve means each comprise an auxiliary valve port located in the low pressure end wall 34, which port is controlled by a poppet type valve member which when in closed position seals against the low pressure end face of the rotor with appropriate running clearance. As will be observed from FIG. 2, the auxiliary valve ports are arranged in peripherally spaced relation concentrically with respect to the axes of the rotors with which they cooperate, at radial distances from the respective axes which correspond to the radii of the respectively cooperating grooves. Also, it is to be noted that the several auxiliary valve means 60 cooperating with the male rotor are peripherally spaced further apart than are the counterpart auxiliary valve means 62 cooperating with the female rotor. The reason for this is that in the em bodiment of apparatus illustrated the male rotor is provided with four lands while the female rotor is provided with six. Thus the female rotor turns at only two-thirds of the speed of the male rotor, and consequently the auxiliary valve ports must be spaced peripherally closer together than the corresponding ports cooperating with the male rotor if the same timing relation between the ports and the grooves passing the ports is to be maintained for both rotors.

As has previously been pointed out, when the rotors are turned (in the direction of the arrows 64, 66 in FIG. 2), the places of intermesh between the lands and grooves of the rotors on the low pressure side of the compressor move axially away from the low pressure end wall to the high pressure end wall to form suction chambers which are initiated at minimum (zero) volume when the places of intermesh are at the low pressure end wall and which progressively increase in volume as chevron shaped chambers as the places of intermesh move to the high pressure end wall, at the plane of which the communicating grooves forming each chamber substantially immediately i move out of mesh with each other so that the grooves are open and unobstructed from end to end thereof and the maximum intake volume has been developed.

If the grooves forming any given chamber are to be filled to their full capacity, the peripheral extent of the inlet port as determined by the positions of the port edges 56 and 58, must be such that the port is open and in communication with the grooves until they have opened up to their full length. With compressors having rotors of the usual or more or less standard construction such as is shown in the present embodiment, the wrap angle of the lands and grooves is such that if the inlet is to remain open until the grooves are fully open, or in the other words have attained full volume during the suction phase of the cycle, the peripheral extent of the inlet port must be substantially that of the valve chamber (seen in REG. 2) which includes not only the main inlet port 38 but also the valve chamber associated with the several auxiliary valve means 60 and 62 and which is in communication with inlet passage 42.

This characteristic of the apparatus is made use of in accordance with the present invention, in order to effect the desired regulation of the capacity of the compressor. To do this, the peripheral limiting edges 56 and 58 of the main port are located so that this port is closed and the flow of working fluid into the suction chamber terminated at a time when the chamber has reached only that fraction of its full volume or capacity which represents the minimum capacity .of the compressor to which it can be regulated. If we now assume that no auxiliary inlet valve means is provided or that any such means is closed, it will be evident that after the inlet ends of the grooves forming the chambers have passed the limiting edges 56, 58 of the main port, further rotation of the rotors to open up the grooves to their full volume, will result only in the further expansion, with concomitant drop in pressure, of the quantity of fluid representing the fraction only of the capacity of the compressor, that has been trapped in the suction chamber. This trapped fluid is carried by the rotors, at the depressed pressure relative to inlet pressure, around to the pressure side of the compressor as the rotors revolve, until the compression chambers previously described are initiated by the coming into intermesh of the lands and grooves on the pressure side of the compressor at the inlet end wall thereof. When the compression chambers are formed, their content is at depressed pressure and as they diminish in volume as the rotors continue to revolve, the content of the chambers first is brought back to initial pressure and thereafter further compressed to the final pressure at which it is discharged to the discharge passage 44 through port 40. While some energy is absorbed by the production of what amounts to a partial vaccum in the suction chambers after the inlet port is closed at part load, the loss resulting from this is substantially less than would be incurred if the same part load capacity were achieved by means of throttling the intake to the suction chambers during the entire period of the suction phase of the cycle.

It, now partial capacity greater than the minimum partial capacity determined by the main inlet port, is desired, then the several auxiliary valve means may be opened to thereby delay cutting off the suction chambers from the inlet, until the chambers have reached whatever larger values of partial capacity that may be desired, up to full capacity that may be reached when all such auxiliary valve means are opened.

Since the groove portions of each rotor which form each of the suction chambers are in communication with each other throughout the suction phase of the cycle, it is immediately apparent that the desired regulation of the capacity of the chambers, which is determined by the position of the rotors at the point of suction cutoff, may be eliected by the use of one set of auxiliary valves, such for example as the set of valves 60 cooperating with the male rotor or the set of valves 62 cooperating with the female rotor. However, as a general rule, the employment of both sets of valves is desirable in order to provide maximum inlet port area for any given capacity and thus reduce losses due to resistance to flow through the inlet into the suction chambers.

In the above described embodiment, it will be evident that the nature of the regulation that is elfected is of a step-by-step character, depending upon the number and location of the auxiliary valves that are opened.

Also it is to be noted that regulation of the apparatus just described, to reduce the capacity of the machine, also operates to alter the compression ratio that is effected. Since the size of the discharge port 40 is fixed, the volumes of the compression chambers at the time of discharge is a constant, whereas the volumes of these chambers at the time when compression is initiated from normal inlet pressure is different, depending upon the degree of capacity regulation that has been applied and the degree to which the normal inlet pressure has been reduced to a depressed A pressure in the suction chambers following the cutoff of these chambers from the inlet conduit. If constant or any other selected variation in the pressure ratio is desired in conjunction with capacity regulation effected by employment of the present invention, this may readily be accomplished by well-known means for regulating the exhaust port of the compressor, which forms no part of the present invention.

FIGS. 3 and 4 illustrate another embodiment of compressor embodying the invention, which differs from that previously described only in the kind of auxiliary inlet valve means employed. In the present embodiment, in-

stead of using poppet valves, slide valves 68 and 74) are employed, the faces of which are in sealing proximity of the end faces of the rotors when the valves are closed. These valves are arranged to move radially to uncover valve ports similar to the valve ports for the poppet valves shown in the modification illustrated in FIG. 1, and since the present embodiment is illustrated as having rotors with four and six lands respectively, the peripheral spacing of the several auxiliary valve means is the same as that previously described. In this modification also, step-bystep regulation of the capacity of the compressor is effected as the several different auxiliary valve means are opened or closed.

The modification illustrated in FIGS. 5 and 6 also makes use of slide valves but is capable of etfecting continuous progressive regulation rather than step-by-step regulation of the kind effected by the embodiments herein before described.

As will be observed more clearly from FIG. 6 the main inlet port 38 extends transversely across the bottom of the low pressure side of the end wall of the compressor, and from this main port two auxiliary ports which are in elfect peripheral extensions of the main port, extend upwardly and tangentially of the rotors to accommodate the vertically and generally tangentially movable slide valves 72 and 74. As will be apparent from the drawings, regulation from substantially full capacity which is obtained with the valves shown in the positions illustrated in FIG. 6, to partial capacities down to the minimum regmlable capacity, may be gradually and progressively efiected by moving the valve members 72 and 74 downwardly from the positions shown in FIG. 6 until they reach the lower ends of their travel to close the auxiliary portions of the inlet port means.

FIGS. 7 and 8 illustrate another embodiment which is capable of providing continuous progressive regulation. In this embodiment the main inlet port 38 is located entirely in the end wall of the compressor and is provided with a fixed edge 58 for determining the point of cutoif of the entrance of working fluid into the grooves of the male rotor 20. The point of cutoff of the working fluid into the grooves of the female rotor 24 is determined by the movable edge 78 of a rotary slide valve 76 movable with respect to a suitable auxiliary valve port in the end wall of the compressor. Obviously as the valve 76 is turned clockwise as viewed in FIG. 8 the capacity of the compressor is increased from minimum regulable capacity which is elfected with the valve in the position shown in the figure to maximum capacity.

The modification shown in FIGS. 9 and 10 is generally similar to that shown in FIGS. 7 and 8 but dilfers therefrom in that the main inlet port 38 is located partly in the end wall and partly in the barrel portion of the compressor, and further in that the rotary slide valve 80 is concentrically movable around the axis of the male rotor 20 to cause the limiting edge 82 of the valve to move toward or away from the fixed edge 56 to regulate the capacity of the compressor. In this embodiment, with the regulating auxiliary valve 80 located around the axis of the male rotor, a main inlet port having a portion opening radially through the barrel portion of the housing is desirable in order to avoid any constriction of the inlet port when the auxiliary valve 80 is moved to full capacity position, in which case, as will be evident from FIG. 10, the end edge of the valve member 80 opposite the control edge 32 will move downwardly into the port area.

While in all of the modifications in which the auxiliary valve members move rectilinearly, it is evident that such valves may be used in conjunction with either one or both of the rotors, it is apparent that in the case of the modifications utilizing rotary valve members, a single valve member, cooperating with either the male or the female rotor as desired, provides the most practical construction.

From the foregoing, it Will be evident that the invention is capable of being embodied in many different specific forms of apparatus and it is accordingly to be understood that the invention is not limited to' the forms of construction herein disclosed by Way of example, but is to be considered as embracing all forms of apparatus fall ing within the scope of the appended claims.

I claim:

1. A rotary piston, positive displacement, elastic fluid compressor comprising a housing structure providing a barrel portion having intersecting bores with coplanar axes and a low pressure end wall further providing an inlet passage terminating in a main inlet port communicating with one end of said bores on one side of said plane constituting the low pressure side of the compressor and a high pressure discharge port communicating with said bores on the opposite side of said plane constituting the high pressure side of the compressor at a place axially spaced from said low pressure end wall, male and female rotors provided with intermeshing helical lands and grooves having an efiective wrap angle of less than 360 rotatably mounted in said bores and operative to sequentially form with the confronting portion of the housing structure on said low pressure side, a series of chevron shaped suction chambers expanding from mini mum volume at the inlet ends of said rotors to maximum volume as the places of intermesh forming the apexes of said chambers move axially away from said inlet ends as the rotors revolve until the lands and grooves move out of mesh at a fixed transverse plane am'ally spaced from said inlet ends and at which plane said discharge port is located, said lands and grooves subsequently intermeshing at the inlet ends of the rotors on said high pressure side as the rotors revolve to sequentially form with the confronting portion of the housing structure a series of chevron shaped compression chambers running out to minimum volume at said transverse plane as the places of intermesh forming the apexes of said compression chambers move axially from the inlet ends of the rotors to said transverse plane, the size of said main inlet port being so limited peripherally that the inlet ends of the grooves forming any given suction chamber pass out of registry with said main inlet port to close the chamber before the grooves forming the chamber move out of d mesh at said transverse plane and with the rotors in a position such that the volume of the chamber is that fraction of the maximum volume which corresponds to the minimum desired partial capacity of the compressor,

and regulating means for controlling the capacity of the compressor between said minimum partial capacity and maximum capacity comprising auxiliary inlet port means in said low pressure end wall located to register with each of the grooves of at least one of said rotors in positions of the groove out of communication with said discharge port from the position at which the inlet end of the groove has just passed out of registry with said main inlet port to the position at which the cooperating land determining the apex end of the suction chamber has passed out of mesh with the groove, and auxiliary inlet valve means capable of selective adjustment during operation of the compressor for controlling said auxiliary inlet port means to regulate the capacity of the compressor.

2. A compressor as defined in claim 1 including conduit means for placing said inlet passage in communication with said auxiliary inlet port means. I

3. A compressor as defined in claim 1 in which said regulating means comprises a selectively movable auxiliary inlet valve member for controlling each auxiliary inlet valve port.

4. A compressor as defined in claim 3 in which each said valve member is of the rectilinearly movable type.

5. A compressor as defined in claim 4 in which each said valve member is of the poppet type.

6. A compressor as defined in claim 5 in which said auxiliary inlet port means comprises a plurality of ports spaced peripherally about the axis of at least one of said rotors.

7. A compressor as defined in claim 5 in which said auxiliary inlet port means comprises a plurality of ports spaced peripherally about the axes of each of said rotors.

8. A compressor as defined in claim 4 in which each said valve member is of the slide valve type.

9. A compressor as defined in claim 8 in which each said valve member is movable generally radially with respect to the axis of the associated rotor.

10. A compressor as defined in claim 9 in which said port means comprises a plurality of ports peripherally spaced about the axis of at least one of said rotors.

11. A compressor as defined in claim 8 in which said port means comprises a plurality of ports peripherally spaced about the axes of each of said rotors.

12. A compressor as defined in claim 3 in which said regulating means comprises an elongated auxiliary inlet port extending generally peripherally of at least one of n u said rotors and a slide type auxiliary inlet valve member for controlling said port.

13. A compressor as defined in claim 12 in which said auxiliary inlet port communicates with and forms a peripheral extension of said main inlet port, whereby movement of said auxiliary inlet valve member between closed and open positions provides continuous progressive regulation of the compressor between minimum and maximum capacities thereof.

14. A compressor as defined in claim 13 in which an auxiliary inlet port and valve member is provided for cooperation with each of said rotors.

15. A compressor as defined in claim 12 in which said elongated auxiliary inlet port is straight and said auxiliary inlet valve is rectilinearly movable.

16. A compressor as defined in claim 15 in which said auxiliary inlet port communicates with and provides a peripheral extension of said main inlet port.

17. A compressor as defined in claim 16 in which an auxiliary valve port and valve member is provided for each rotor.

18. A compressor as defined in claim 12 in which said elongated auxiliary inlet port is curvedand said auxiliary valve is of the rotary slide type turnable about the axis of the associated rotor.

19. A compressor as defined in claim 18 in which said elongated auxiliary inlet port communicates with and provides a peripheral extension of said main inlet port.

20. A compressor as defined in claim 19 in which said auxiliary inlet valve port and valve member cooperate with said female rotor.

21. A compressor as defined in claim 19 in which said auxiliary inlet valve port and valve member cooperate with said male rotor.

References Cited in the file of this patent UNITED STATES PATENTS 996,169 Van Deventer June 27, 1911 2,266,820 Smith Dec. 23, 1941 2,459,709 Lysholm Jan. 18, 1949 2,480,818 Whitfield Aug. 30, 1949 2,519,913 Lysholm Aug. 22, 1950 2,580,006 Densham Dec. 25, 1951 2,620,968 Nilsson Dec. 9, 1952 2,622,787 Nilsson Dec. 23, 1952 2,642,003 Whitfield June 16, 1953 2,656,972 Rathman Oct. 27, 1953 FOREIGN PATENTS 384,355 Great Britain Dec. 8, 1932 1,158,976 France Feb. 3, 1958 

1. A ROTARY PISTON, POSITIVE DISPLACEMENT, ELASTIC FLUID COMPRESSOR COMPRISING A HOUSING STRUCTURE PROVIDING A BARREL PORTION HAVING INTERESTING BORES WITH COPULAR AXES AND A LOW PRESSURE END WALL FURTHER PROVIDING AN INLET PASSAGE TERMINATING IN A MAIN INLET PORT COMMUNICATING WITH ONE END IF SAID BORES ON ONE SIDE OF SAID PLANE CONSTITUTING THE LOW PRESSURE SIDE OF THE COMPRESSOR AND A HIGH PRESSURE DISCHARGE PORT COMMUNICATING WITH SAID BORES ON THE OPPOSITE SIDE OF SAID PLANE CONSITITUTING THE HIGH PRESSURE SIDE OF THE COMPRESSOR AT A PLACE AXIALLY SPACED FROM SAID LOW PRESSURE END WALL, MALE AND FEMALE ROTORS PROVIDED WITH INTERMESHING HELICAL LANDS AND GROOVES HAVING AN EFFECTIVE WRAP ANGLE OF LESS THAN 360* ROTATABLY MOUNTED IN SAID BORES AND OPERATIVE TO SEQUENTIALLY FORM WITH CONFRONTING PORTION OF THE HOUSING STRUCTURE ON SAID LOW PRESSURE SIDE, A SERIES OF CHEVRON SHAPED SUCTION CHAMBERS EXPANDING FROM MINIMUM VOLUME AT THE INLET ENDS OF SAID ROTORS TO MIXIMUM VOLUME AS THE PLACES OF INTERMESH FORMING THE APEXES OF SAID CHAMBERS MOVE AXIALLY AWAY FROM SAID INLET ENDS AS THE ROTORS REVOLVE UNTIL THE LANDS AND GROOVES MOVE OUT OF MESH AT A FIXED TRANSVERSE PLANE AXIALLY SPACED FROM SAID INLET ENDS AND AT WHICH PLANE SAID DISCHARGE PORT IS LOCATED, SAID LANDS AND GROOVES SUBSEQUENTLY INTERMESHING AT THE INLET ENDS OF THE ROTORS ON SAID HIGH PRESSURE SIDE AS THE ROTORS REVOLVE TO SEQUENTIALLY FORM WITH THE CONFRONTING PORTION OF THE HOUSING STRUCTURE A SERIES OF CHEVRON SHAPED COMPRESSION CHAMBERS RUNNING OUT TO MINIMUM VOLUME AT SAID TRANSVERSE PLANE AS THE PLACES TO INTERMESH FORMING THE APEXES OF SAID COMPRESSION CHAMBERS MOVE AXIALLY FROM THE INLET ENDS OF THE ROTORS TO SAID TRANSVERSE PLANE, THE SIZE OF SAID MAIN INLET PORT BEING SO LIMITED PERIPHERY THAT THE INLET ENDS OF THE GROOVES FORMING ANY GIVEN SUCTION CHAMBER PASS OUT OF REGISTRY WITH SAID MAIN INLET PORT TO CLOSE THE CHAMBER BEFORE THE GROOVES FORMING THE CHAMBER MOVE OUT OF MESH AT SAID TRANSVERSE PLANE AND WITH THE ROTORS IN A POSITION SUCH THAT THE VOLUME OF THE CHAMBER IS THAT FRACTION OF THE MINIMUM VOLUME WHICH CORRESPONDS TO THE MINUMIM DESIRES PARTIAL CAPACITY OF THE COMPRESSOR AND REGULATING MEANS FOR CONTROLLING THE CAPACITY OF THE COMPRESSOR BETWEEN SAID MINIMUM PARTIAL CAPACITY AND MAXIMUM CAPACITY COMPRISING AUXILIARY INLET PORT MEANS IN SAID LOW PRESSURE END WALL LOCATED TO REGISTER WITH EACH OF THE GROOVES OF AT LEAST ONE OF SAID ROTORS IN POSITIONS OF THE GROOVES OUT OF COMMUNICATION WITH SAID DISCHARGE PORT FROM THE POSITION AT WHICH THE INLET END OF THE GROOVE HAS JUST PASSED OUT OF REGISTRY WITH SAID MAIN INLET PORT TO THE POSITION AT WHICH THE COOPERATING LAND DETERMINING THE APEX END OF THE SUCTION CHAMBER HAS PASSED OUT OF MESH WITH THE GROOVE, THE AUXILIARY INLET VALVE MEANS CAPABLE OF SELECTIVE ADJUSTMENT DURING OPERATION OF THE COMPRESSOR FOR CONTROLLING SAID AUXILIARY INLET PORT MEANS TO REGULATE THE CAPACITY OF THE COMPRESSOR. 